The present invention relates to adaptive lasers, and, more particularly, to a method and system for increasing the power deliverable by an adaptive laser subject to wavefront distortions.
Adaptive optical systems have been developed to deal with the loss of power deliverable by a laser due to wavefront aberrations. These aberrations can be introduced by the laser system, for example by imperfections in the optical train. Alternatively, nonuniformities in the laser beam media, for example, atmospheric anomalies, can distort the laser wavefront.
Generally, an adaptive laser includes a laser source with optics adaptable to control the outgoing waveform, a waveform sensor, and a processor for converting sensor data into commands directed to the adaptable optical element of the laser. Ideally, such a system would include a waveform sensor with infinite resolution, a completely deformable mirror, and a processor which validly converts the sensor data to mirror adjustments to produce the desired waveform. In practice, there are limitations to the adaptability of the laser source, to the accuracy, precision and validity of the sensor measurements, and, as a result of the foregoing, to the validity of the translation of sensor data into commands by the processor.
Typically, the adaptable element of the laser source is the primary mirror. A number of actuators can be applied to deform the mirror. The degree to which mirror deformation can be controlled is limited by the number of actuators. Thus, generally, even with ideal waveform determination and translation into actuator commands, the resulting waveform only approximates the desired uniform wavefront.
Of course, available sensors fall short of the infinite resolution waveform ideal in several respects. It turns out that the most practical sensors measure waveform indirectly and with quite limited resolution. Examples of such sensors include the Hartman sensor, the shearing interferometer and the integrated imaging irradiance sensor. The output of these sensors is typically treated as wavefront slope data, which is the derivative of the desired waveform or optical path difference (OPD) data.
There are several substantially equivalent ways for a processor to convert such sensor data into actuator commands. In one approach, the slope data is integrated to obtain a reconstructed wavefront. The resulting OPD data can then be processed according to a predetermined relationship between actuator commands and effects on OPD so as to minimize the root-mean-square of the expected OPDs.
Another approach bypasses the conversion to OPD data by using a direct relationship between actuator commands and effects on wavefront slope. This approach applies a least squares fit of wavefront slopes to a discrete wavefront map, thereby attempting to minimize the sum of squares of the wavefront slope measurements across the laser aperture. The applied algorithm has the form: EQU a=(H.sup.T H).sup.-1 H.sup.T s
where a is the actuator command vector estimate, H is the actuator-sensor coupling matrix, and s is the slope vector. The actuator command vector estimates the commands predicted to result in the desired wavefront figure of merit at the sensor. The actuator-sensor coupling matrix, also known as the actuator-sensor Jacobian, describes what slope value a given subaperture will measure when a given actuator is pushed.
In practice, adaptive lasers in accordance with the foregoing fall far short of theoretical performance. In some situations, not only is the ideal not approached, but the "corrected" laser delivers less power than would the same laser without the correction. Furthermore, successive corrections can fail to converge and introduce dynamic instability into the system.
It is anticipated that advances will permit greater numbers of actuators and improved sensors. However, the performance of future systems incorporating such refinements and present systems incorporating today's technology are not optimized by the current methods of translating waveform data into actuator commands. A need exists for a new adaptive optics system and corresponding method for the enhanced power delivery by lasers.